New Lightwave Modulator

Yesterday I completed the construction of the crystal-controlled tone generator which will be used to modulate my lightwave transmitter during future clear-air / cloudbounce tests.




It was installed on the lightbox, right beside the original 556 CW beacon / tone generator.




The crystal-controlled oscillator uses a CD4060 IC as an oscillator-divider and produces a ~550Hz or a ~1098Hz squarewave from the 4.5MHz crystal.

4500KHz xtal divided by 8192 showing 549Hz output
The original 556 tone generator will be kept for aural CW and beacon modes as it provides a stable enough signal for this purpose but for the very narrow bandwidths that I plan to use when digging into the noise with Argo, I reasoned that the signal needed to be more stable.

As can be seen by comparing the two oscillators (crystal on the left and 556 on the right), the 556 has a lot of drift (although it looks like it might eventually stabilize) and, as well, produces several spurious signals ... probably robbing power from the main tone. The crystal-controlled signal is rock solid and doesn't appear to generate any parasitic signals in the process. The trace below the crystal signal is unrelated to the oscillator.




When I first wired the unit up, I found an unstable low frequency oscillation from the 4060 during key-up conditions, due no doubt, to the lengthy leads inside the box. This was cured by adding a pull-up resistor to the keying line as shown in the final schematic below.












Now it's on to building another fresnel-lens receiver box which will be needed for any field work here on the island.
Steve McDonald, VE7SL, is a regular contributor to AmateurRadio.com and writes from British Columbia, Canada. Contact him at [email protected].

3 Responses to “New Lightwave Modulator”

  • w3fis:

    Can you suggest a source for parts?

    73 /paul W3FIS

  • Steve VE7SL:

    Paul …. my first choice is always Digikey because of their inexpensive fast shipping. Second choice is Mouser.

    Steve

  • Mike, WV2ZOW:

    Paul: Have you tried modulating the light with RF? Maybe 500 kHz. Use cw, and a sensitive receiver on the other end? Might be easier to copy, as it would eliminate all the hum and other light modulation you may be hearing. –Mike, WV2ZOW

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